The invention relates generally to the fields of seismic and acoustic detection of surface disturbances. It is concerned more particularly with the integration of the two types of detection means in a single field or air launched unit. The invention also relates to the field of control circuitry used to discriminate between true and false signals.
The need for passive military intelligence about concealed enemy ground activity has encouraged the development of various "listening" systems which monitor the level of vibrations in specified areas. The information gained can serve to corroborate other forms of intelligence or, if reliable enough, electronic detection may form the sole basis for a command decision. In either case, if higher quality information is obtained, there is less likelihood of unintended encounter with the enemy and more chance of avoiding casualties and loss of time and material. When the aim is not to avoid but to locate the enemy, electronic devices can detect even a relatively small build-up of men or material.
One of the most valuable uses of military listening devices is in detecting the movement of motorized equipment by the enemy. In general, the operation of motorized equipment produces two types of vibrations. The first type, commonly referred to as seismic disturbance, is inaudible mechanical vibration induced by movement of the object. Seismic waves are transmitted through solid structure such as the earth. The second type of vibration, commonly referred to as sonic or acoustic waves, is associated with the audible noise produced by the object and is transmitted primarily through the air. In the past, remote detection and classification of motorized equipment was accomplished by monitoring either the seismic or the acoustic vibrations produced. Probes of either kind were planted manually or airdropped into the region under surveillance. The transmitted signals, if any, were received by a remote listening post.
A major problem encountered by prior art devices was minimizing the number of false alarms. Instead of detecting only the movement of motorized equipment, seismic sensing devices produced unwanted signals on ordnance detonations, tree root movement, animal movement, or nearby aircraft transits. Acoustic detection devices had similar difficulties in discriminating between noise produced by motorized equipment on the ground and transient noise from other sources. In particular, if ground vehicles were the target, passing aircraft would frequently cause false alarms. The inaccuracies of the prior art equipment seriously increased the uncertainty in detection and degraded the willingness of field commanders to rely on such information in determining the status of an area which might be held by the enemy. The high false alarm rate of the equipment also decreased the useful lifetime of a particular unit since the battery-powered transmitter spent more time in the "ON" condition than it would have if it had been reporting detection of target vehicles alone. This in turn added to the cost of the equipment and often required dangerous redeployment of seismic or acoustic probes in the same area. Moreover, a high false alarm rate of units in a designated area might overload the r.f. transmitting band being used.